The present invention relates to a device that controls elevator car speeds. More particularly, the invention relates to a centrifugally actuated governor.
A common challenge in elevator design is engineering safety systems to prevent or react to elevator malfunction. One such safety system is the speed governor. Elevator speed governors are designed to prevent elevator cars from exceeding a set speed limit. The governor is a component in an automated safety system, which is actuated when the elevator car exceeds a set speed and either signals a control system to stop the car or directly engages safeties to stop the car. One commonly known governor is a centrifugally actuated governor.
A common design of centrifugal governors used in elevator systems employs two masses connected kinematically in an opposing configuration by links and pinned to a tripping sheave rotating about a common axis. These interconnected parts create a rotating mechanism whose angular velocity is common with the sheave. The angular velocity of the rotating masses results in a centrifugal force acting to propel the masses away from the sheave axis of rotation. A rope loop wrapped partially around the sheave located at one end of the elevator hoistway, connected to the elevator car, and wrapped partially around a tensioning sheave at the opposite end of the hoistway ensures that the elevator car speed is related to the sheave angular velocity. In another commonly known design, the governor is mounted to and moves with the car. This implementation may use a static rope anchored at the top and bottom of the hoistway and wrapped partially around the tripping sheave and an adjacent idler sheave.
As the governor masses pivot about their pinned locations on the sheave, the moment of inertia of the masses changes as a function of angular velocity. The radial outward movement of the masses is limited by a device that prevents mass movement up to a set elevator car speed. The movement of the masses is typically controlled by the use of a spring connected between the sheave and one of the masses. The purpose of this arrangement is to create a spring force proportional to the extension of the spring and its inherent spring constant, which resists the centrifugal force generated by the angular velocity of the rotating sheave. The spring force maintains a controlled relative position between the masses and the sheave. Controlling the spring force as a function of the centrifugal force together with the geometry of the mechanism allows actuating the governor by controlled outward movement of the mechanism in the radial direction.
There are several limitations to using a spring connection to control the radial outward movement of the masses. First, the combination of spring and rotating inertia of the masses results in a natural frequency of vibration, which might overlap with the natural frequency of the elevator system. Overlapping natural frequencies, combined with an excitation force, for example if someone in the elevator car jumps, bounces, or rhythmically rocks the car, can cause a vibration response in the governor and thereby falsely trip the governor below a set elevator car speed. Second, this design approach requires accommodation for the manufacturing tolerances of the spring and its attachment means. Low cost commercial springs can have a wide range of spring constant tolerances, which requires spring length adjustment or pre-tensioning the spring to avoid distributions in the spring force and thereby in performance of the governor. Metal springs, which are typically used because of commercial availability and cost, have other limitations including potential spring constant changes after repeated compression/extension and susceptibility to corrosion. Polymer springs can be expensive to produce, have limited performance due to weaker material properties, are less commercially available, and can have higher tolerances.
In light of the foregoing, the present invention aims to resolve one or more of the aforementioned issues that afflict conventional governors.